1. Field of the Invention
The present invention is directed to blowout preventers, to actuators for blowout preventer rams, and to methods of their use.
2. Description of Related Art
In a variety of situations, blowout preventers are used to control sub-surface pressures that may adversely affect equipment used in drilling oil and gas wells. Manual mechanisms and pneumatic or hydraulic pressure are employed to act on a piston to close or open ram sealing elements. Often hydraulic actuation is used when the required closing forces are relatively high. Hydraulic actuation force is applied to a cylinder containing a piston which in turn acts on a shaft having a ram element connected thereto. A closing force in such an apparatus may be substantially equivalent to the effective cross-sectional area of the piston multiplied by the pressure of the hydraulic fluid.
In a variety of prior art blowout preventers an enhanced closing force is applied to rams that are part of the blowout preventer, e.g., shear rams for shearing and closing off a tubular. In some prior art systems to achieve a desired closing force an hydraulic booster increases the effective closing force for a given hydraulic actuation pressure. In certain prior art systems, an hydraulic booster piston is placed in series with a main actuator piston and often the hydraulic booster provides a piston which has a larger cross-sectional area upon which the hydraulic pressure acts, thereby increasing the closing force. In one aspect a booster piston is attached to a far end of a guide rod and the near end of the guide rod acts on a high pressure side of the main actuator piston. A net closing force on the primary piston shaft is increased by the mechanical force to the main actuator piston resulting from hydraulic pressure to the booster piston. On some prior art systems the additive force of a booster or secondary piston on a primary piston will produce a total force that exceeds the strength of material of a ram block, resulting in the yielding or bending of the material. e.g. material of the wall on either side of a top seal vertical leg component.
U.S. Pat. No. 5,575,452 discloses, inter alia, a blowout preventer ram actuator mechanism with a primary piston including an outer sleeve portion which supports an independently movable locking piston which has tapered surfaces, and locking segments each engage one of a plurality of tapered locking rods fixed to the actuating mechanism housing. Since locking piston components move independently of the primary piston, an axially centered boosting force may not be exerted directly against internal moving parts without risking premature locking of the primary piston.
U.S. Pat. No. 6,244,560, co-owned with the present invention, discloses, inter alia, blowout preventer ram actuating mechanisms that include an hydraulic booster for enhancing the ram closing force. The ram actuating mechanism may be compatible for use with primary pistons which include internal moving components, such as self locking pistons. The ram actuating mechanism provide an hydraulic booster without increasing the diameter of the booster pistons above the diameter of the primary piston, so that stack height need not be increased to accommodate a relatively large diameter hydraulic booster. The ram actuating mechanism may utilize the same piston housing as used by the primary piston, and the booster pistons may act mechanically in series upon the primary piston to increase axial ram closing force.
The present inventor has recognized that there is a need to reduce the overall space and volume required by a blowout preventer and to reduce the weight of blowout preventers; but it is also necessary that a blowout preventer develop sufficient force on its rams to shear a tubular about which it is positioned. The present inventor has also recognized that it is also desirable in some circumstances to relieve or reduce the force on blowout preventer rams to reduce the pressure that is initially applied to the ram bodies and to their seals by a dual-piston actuator in order to prolong seal life and/or prevent deformation of ram blocks. The present inventor has also recognized that, in reducing the pressure on the closed rams, the requirement remains to positively maintain the rams in a closed position.